The change to the illuminated side markers started on Jul. 31, 1969 with VIN 011948 (plus 011784, 011830, 011899,
011903, 011912-011916, 011918, 011920, 011921, 011930-011933, 011941-011945) when lots of other changes were made. For example, locked ignition key in position 1, gas tank evaporation, 55 Amp alternator, etc.
During the repainting of my 280SL we took the non-illuminated side markers off and it was then that I noticed that one had a mounting screw and the others did not. I asked Dave Gallon about this and he mentioned that the early versions without the screw had 110 826 XX 41 numbers that were replaced sometimes in late 1968 with the non-illuminated 000 826 XX 41 series with the screw. All illuminated versions do have the screw.